A message written on a banner at a vigil on the Boston Common on Tuesday. / Stan Honda, AFP/Getty Images

by Donna Leinwand Leger, Kevin Johnson and Aamer Madhani, USA TODAY

by Donna Leinwand Leger, Kevin Johnson and Aamer Madhani, USA TODAY

BOSTON - As a small army of federal and local law enforcement authorities launched a broad investigation into the deadly bombings that ripped through crowds gathered at the finish line of the Boston Marathon, there were many more questions than answers.

Officials, including President Obama who will be traveling to the city Thursday for an interfaith memorial service, acknowledged Tuesday that it was not known whether the attack was the work of international or domestic terror groups - or perhaps a lone wolf who suddenly burst on the scene from somewhere far off the national grid.

Neither was there an immediate motive for an assault that now takes its place on the calendar as another terrible April day in America, alongside the grim anniversaries for Oklahoma City, Columbine and Virginia Tech.

Yet from the random pieces left on an asphalt killing field, which Boston Police Commissioner Ed Davis described as the "most complex crime scene in the history or our department," authorities believe they have learned critical information about the makeup of devices that left three dead and 176 others wounded.

The devices, according to Boston FBI chief Richard DesLauriers, are believed to have been assembled in common kitchen pressure cookers filled with BB pellets and nail fragments that cut a bloody swath of destruction through crowds of spectators at the iconic race site.

DesLauriers said late Tuesday afternoon that the "heavy" explosives were believed to have been concealed in "dark-colored nylon backpacks" carried and placed near the finish line where they were detonated as crowds cheered runners streaming across the finish line.

The FBI official said the bomb fragments have been shipped to the agency's laboratory in Quantico, Va., for further review, but DesLauriers cautioned that the inquiry was "still in its infancy'' and that there were no immediate suspects in the attack.

"The range of suspects and motives is wide open," said DesLauriers, whose agency is leading a far-flung probe involving some 30 law enforcement agencies and 1,000 investigators.

Though there have been no immediate arrests, he said authorities were moving "methodically, carefully yet with a sense of urgency."

"Someone knows who did this," DesLauriers said.

A separate law enforcement official, who was not authorized to comment publicly, said gunpowder also was used in the explosives' construction. DesLauriers and Gene Marquez, Bureau of Alcohol, Tobacco, Firearms and Explosives director in Boston, declined to comment on the presence of gunpowder. Another law enforcement official, also not authorized to comment publicly, said circuit board material that may have assisted in the use of a timing device was among the items recovered from the crime scene.

Crude pressure cooker devices were the subject of separate U.S. Department of Homeland Security bulletins in 2004 and 2010, outlining their "frequent" use in Afghanistan, India, Nepal and Pakistan. While not commonly deployed in the U.S., at least one of the devices recovered in the foiled May 2010 attempted car bombing in New York's Times Square "incorporated a pressure cooker" along with fireworks, according to the 2010 DHS bulletin.

"The Department of Homeland Security is issuing this information bulletin to alert frontline border inspectors and agents ... that there is continued interest by terrorist organizations to use innocuous items to package improvised explosive devices (IEDs)," the 2004 bulletin stated. "The technique commonly taught in Afghan terrorist training camps is the use/conversion of pressure cookers into IEDs."

Instructions for similar devices also have been published by al-Qaeda in its English-language Inspire magazine, which has repeatedly urged U.S. residents to take up arms against the government.

Despite the stylistic links to al-Qaeda and extremists in Afghanistan, U.S. investigators, however, have refrained from attributing the attack to any one terror group in part because the bomb-making information has been circulated so broadly.

"Our mission is clear," DesLauriers said earlier Tuesday, "to bring to justice those responsible for the marathon bombing. ... We will go to the ends of the earth to find the subjects responsible for this crime."

While no groups or individuals have claimed responsibility for the crime, the power of the devices was described in vivid detail by physicians who tended to the victims. Dr. George Velmahos, chief of trauma surgery at Massachusetts General Hospital aid some of the injuries were "traumatic amputations," in which the blasts nearly blew off one victims' legs.

Velmahos said Tuesday that four victims have so far lost limbs in the attacks.

The explosions 12 seconds and 100 yards apart ripped into the crowds gathered Monday afternoon for the finale of the traditional marathon that wound through the streets of Boston.

Velmahos had said that all of the victims had "10, 20, 30, 40 pieces of shrapnel embedded in their bodies, mostly in their legs, but as high up as their necks." He had described the shrapnel as pea-sized pellets and nails stripped of their heads.

Massachusetts Gov. Deval Patrick emphasized Tuesday that the two exploded devices were the only ones involved in the attack. He said no unexploded devices were found.

At a briefing hosted by federal and local authorities, officials declined to elaborate on the course of their investigation, but they did issue an urgent appeal for the public to come forward with videos and photos taken near the crime scene to help assist possibly in identifying those responsible for the blast. Already, authorities have been provided hours of video and thousands of photographs. DesLauriers said additional forensic teams were being dispatched to the city to assist in analyzing the growing mountain of material.

Investigators also were dispatched to the airport to ask departing race participants and tourists for any photos or video they may have taken near the bomb scene.

Emphasizing the potential importance of the material, Davis said the crime scene - because of its proximity to the marathon finish line - may have been the "most photographed location" in the nation Monday. Immediately after the bombing, Davis said officers seized surveillance video from an undisclosed number of security cameras at businesses throughout the 12-block crime scene area.

In addition, hundreds of potential witnesses had been interviewed in the hours after the blasts, including a Saudi national who investigators closely questioned at a local hospital where he arrived after leaving the bombing scene. Two law enforcement officials said he was not considered a suspect.

Though the officials said they did conduct a search of an apartment connected to the man, it did not produce information of value to the inquiry. "I don't think we've completely closed the book, but it is beginning to look like there is less there," the official said referring to the Saudi man.

The official said a number of leads are being pursued, including an image of a man with multiple backpacks near the scene of the bombings. But the official said "nothing stands out at this point.''

"I think it's going to take a little while," the official said.

"We were lucky that the bomber was not more sophisticated and did not create a more powerful bomb," said Bradley Buckles, who served as the director of the Bureau of Alcohol, Tobacco, Firearms and Explosives from 1999 to 2004.

Buckles, who oversaw the bureau's 9/11 response, compared the Boston bombing to the 1996 Olympic bombing in Atlanta. In that case, Eric Rudolph left a knapsack that later exploded, killing one person and injuring more than 100. "These types of devices can be constructed by almost anyone," Buckles said. "It doesn't require a great deal of sophistication. It's fairly easy to learn the technology of developing and manufacturing these devices."

On Tuesday, meanwhile, Ehsanullah Ehsan, a spokesman for the Taliban in Pakistan, told the AFP news agency that it was not involved in the attack. The Pakistani Taliban had claimed responsibility for the 2010 Times Square bomb plot.

The Boston Marathon bombing is the eighth attack against a marathon and the first in the United States, according to report released today by a federally funded research group at the University of Maryland.

The most recent attack occurred in April 2008 near the crowded starting line at a marathon in Colombo, the largest city in Sri Lanka. The suicide bombing killed 14 civilians and injured 83 people, according to the National Consortium for the Study of Terrorism and Responses to Terrorism. The attacks in Colombo and in Boston are the only ones to kill people at a marathon. Three attacks connected to the Belfast marathon in Northern Ireland occurred between 1998 and 2005.